Answer:
6
Step-by-step explanation:
Let line WY be the line of reflection
RR has the midpoint at Z.
In other words, the line:
= 
SS is another line
It has a midpoint at X. In other words, line SX = 
If SS = 12, then the line SX = 
Because the shape is a parallelogram, it means that RZ = SX = 6